1. Field of the Invention
This invention relates to a seat reclining mechanism
2. Description of the Prior Art
Seat reclining mechanisms are available for vehicle seats, which provide for angular adjustment of the seat proper and the backrest by rotation of a gear in a gear arrangement which is self-locking in the adjusted position.
Such a mechanism is described in U.K. patent application No. 1,091,944. Its construction includes two hinged plates with an internally toothed ring gear on one plate and a spur gear mounted for eccentric motion on the other plate and engaged with the ring gear. Adjustment is achieved by a difference in the numbers of teeth on the two gears. A disadvantage of this construction is that the plates move eccentrically, this movement being transmitted to the backrest, in use, the backrest making what has been termed as "waltzing" movement.
Another construction of a seat reclining mechanism is described in U.K. Patent specification No. 1,170,708 to Recaro. Recaro's disclosure provides a ring gear on each plate and two spur gears which are rotated simultaneously and eccentrically and engage respectively the two ring gears. This construction avoids the "waltzing" disadvantage, but is more expensive to produce. Another embodiment uses a small gear carried on an arm to engage the ring gears. This construction is insufficiently strong, since a large force is carried by the teeth of a small gear. The small gear or its carrying arm is liable to break, or the small gear will be forced out of a locking position, so that the bracket collapses.
Disclosed in the German Patent specification No. 1,555,711 is another structure of a seat reclining mechanism. This construction also has ring gears on the two hinge plates, but a gear carrier is provided mounting four small gears on a circle. Each of these small gears is a composite gear formed of two integral gears having their teeth offset. One integral gear of each of the four composite gears is engaged with one ring gear and the other integral gear of each composite gear is engaged with the other ring gear. The gear carrier is rotated so that the composite gears roll along the ring gears and rotation is achieved by a further gear engaged with external teeth on the gear carrier. The composite gears are very difficult to make and consequently very expensive as one of the integral gears contains eight teeth, while the other integral gear contains only seven teeth. Accordingly, it is also impossible to use a single central control gear to engage fully both of the integral gears of each composite gear. This construction also suffers the disadvantage of having a total of twelve gears, including the carrier, making the mechanism as a whole very expensive to produce.
U.S. Pat. No. 3,972,563 to Gustaffsson, discloses a seat-back tilt adjustment mechanism wherein ring gears are provided on the two hinge plates. In one embodiment there is one planetary gear mounted on a carrier and engaged with both ring gears. This works theoretically, but is impractical for the same reasons as the Recaro construction having one planetary gear, i.e., it would be insufficiently strong to support the force exerted by a person leaning on the backrest of the seat. Gustaffsson also describes an arrangement wherein the carrier carries three planetary gears. This is also impractical for other reasons. There has to be a difference of three teeth between the two ring gears and with the construction illustrated there should be a movement of 30 degrees between the plates for each revolution of the control knob. The force needed to do this is far too large for an average person. Furthermore, the arrangement would not be self-locking and the backrest would collapse in use. A large increase in the number of teeth on the ring gears would avoid these problems, by reducing the angular movement of the hinge plates per revolution of the control knob, but then the teeth would be too weak for the purpose and also the gears would be far too expensive to produce. Cheaper methods such as fine blanking techniques not being possible.
The above and other problems inherent in the prior art are solved by the present invention which provides an improved seat reclining mechanism.